An natural rubber latex is obtained as a sap from a Brazilian rubber tree (Hevea Brasilensis), and contains about 60% of water, about 30% of a rubber component and several % of a protein, a lipid and a mineral, which is normally referred to as a "field latex". On the other hand, natural rubber latex as an industrial raw material is fed as a purified natural rubber latex which is obtained by purifying with a concentration of the rubber component to about 60% and adding 0.2 to 0.7% of ammonia to the latex to prevent putrefaction of the latex.
As the method of purifying the latex by concentration, a centrifugal separation method with a high purification efficacy is generally used. According to this method, purified natural rubber latex is produced by separating a cream-like rubber component formed by centrifugal separation and redispersing the rubber component in water.
However, the serum formed during the centrifugal separation contains some rubber component having a small particle diameter which can not be recovered as the cream component. Therefore, it is required to recover and use such a rubber component in view of the production cost
It is known that the rubber component having a small particle diameter contained in the serum is a fine particle having a particle diameter of about 0.1 to 0.5 .mu.m, and the film-forming property and film strength of a latex film can be improved by adding the rubber component to a normal natural rubber latex. Accordingly, it is a considerably important object to recover the particulate natural rubber in the serum, from the viewpoint of overall production cost.
As the method of recovering the particulate natural rubber from the serum, for example, (A) a method of coagulating a rubber component in a serum using an acid to produce a skim block rubber and (B) a method of adding an acid and a nonionic surfactant to a serum to obtain an acidic latex (British Patent No. 743139) have heretofore been suggested.
However, both of the above methods (A) and (B) have a problem that, since the particulate natural rubber recovered from the serum contains a large amount of impurities, the quality is poor. Also, there is a problem in that, since the latex obtained by the above method (B) is acidic, the latex is limited to special use and has poor utility.
On the other hand, performances such as high electrical insulating properties, low water absorption and being allergy free, and the like, have recently been required of natural rubber products. In order to realize these features, it is required to highly deproteinize the particulate natural rubber.
As a method of obtaining a deproteinized particulate natural rubber, for example, (C) an enzyme is added to the serum to deproteinize the serum, followed by adding an acid, washing the coagulated rubber component with water and drying the rubber component to obtain a low-protein (about 0.4%) solid natural rubber (British Patent No. 695813). However, according to this method, it is difficult to obtain a particulate natural rubber which is allergy free and which contains as small amount of impurities.
Accordingly, it is also considerably important to recover the particulate natural rubber contained in the serum in the state of being deproteinized in view of a realization of high electrical insulating properties, low water absorption and allergy free properties as well as a reduction of the production cost.